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    Re: Lunar eclipse report
    From: Frank Reed CT
    Date: 2004 Oct 28, 17:39 EDT
    Alex E wrote:
    "One problem is of course, to what precision could
    Hipparchus-Ptolemy-Columbus measure their local time
    (with an astrolabe, I suppose)."

    All you need is a careful observation of the sidereal time (which is the right ascension of the zenith). With a fixed installation, you could mount a sight tube. For a temporary setup, you could hang a couple of plumb lines and stand between them extending the lines. This should give local sidereal time within two minutes or better. Assuming roughly two minutes error in eclipse event timings and two minutes error in local time, the average expected error would be about three minutes --less than a degree error in longitude. IF the need had arisen and IF thousands of astronomical teams could have been dispatched safely to distant locations for multiple lunar eclipses, the world could have been mapped in longitude with good accuracy thousands of years ago.

    And:
    "Another problem was the precision of their almanacs.
    (Columbus used "Regiomontan(us) tables". What Ptolemy used,
    I don't know, not speaking of Hipparchus)."

    For this technique, it doesn't necessarily matter. As long as you can predict the date of the eclipse, you're all set. You collect the data from your teams around the world and then compare the results later. This also applies to lunars. The requirements for map-making and active navigation are different. IF sextants had been perfected a few decades earlier and IF someone had realized the value of a good map to future navigation, explorers could have conducted lunar distance observations at every stopping point long before the almanacs had predicted lunars in them. The Moon's position, observed continuously in observatories back home, could be used after the voyage to convert the observations to longitudes. Unfortunately, this sort of observation would have had no negative consequences for the observer so there would be little opportunity to catch observational errors or encourage quality.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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